1. Field of the Invention
The invention relates to a method for processing steel slags and iron carriers such as, e.g., electric furnace slags, converter slags, fine ores, dusts obtained from the production of and, millscale, for recovering pig iron and environmentally safe slags.
2. Description of Related Art
From WO 96/24696 a method for producing pig iron or steel and cement clinker from slags has become known, in which iron- oxide-containing molten slags such as, for instance, steel making slag was mixed with iron oxide carriers and lime, whereupon a ferritic slag was formed. After this, the ferritic slag was reduced in a reduction reactor while forming an iron bath and a sinter phase upon combustion of carbon, whereupon the sinter phase was discharged as a clinker.
Slags have a relatively low thermal conductivity and a heat capacity exceeding that of iron by about 1.5 to 2 times. Essential to the economy of such a method is the heat transfer or so-called post-combustion degree that is obtainable. The post-combustion degree is defined as follows:       Post    ⁢          -        ⁢    combustion    ⁢          xe2x80x83        ⁢    degree    =                              CO          2                +                              H            2                    ⁢          O                            CO        +                  CO          2                +                  H          2                +                              H            2                    ⁢          O                      ⁢          xe2x80x83        ⁢    gaseous    ⁢          xe2x80x83        ⁢    phase  
So far, only insufficient post-combustion degrees have been ensured by the known modes of procedure. Also the             heat      ⁢              xe2x80x83            ⁢      tranfer        =          1      -                                    H            g                    -                      H            b                                    H          pc                                H      g        ⁢          xe2x80x83        ⁢    …    ⁢          xe2x80x83        ⁢    gas    ⁢          xe2x80x83        ⁢    enthalpy    ⁢          xe2x80x83        ⁢          (              at        ⁢                  xe2x80x83                ⁢        gas        ⁢                  xe2x80x83                ⁢        temperature            )                  H      b        ⁢          xe2x80x83        ⁢    …    ⁢          xe2x80x83        ⁢    gas    ⁢          xe2x80x83        ⁢    enthalpy    ⁢          xe2x80x83        ⁢          (              at        ⁢                  xe2x80x83                ⁢        melting        ⁢                  xe2x80x83                ⁢        temperature            )                  H      pc        ⁢          xe2x80x83        ⁢    …    ⁢          xe2x80x83        ⁢    combustion    ⁢          xe2x80x83        ⁢    enthalpy    ⁢          xe2x80x83        ⁢          (              at        ⁢                  xe2x80x83                ⁢        melting        ⁢                  xe2x80x83                ⁢        temperature            )      
does not suffice for an economic mode of operation in the known modes of procedure.
Thermal efficiencies of far more than 70% are achieved neither in conventional blast furnace technologies nor in other processes such as, for instance, fluidized bed processes. Thus, it has already been known to blow into a fluidized bed, together with coal, prereduced and at least partially preheated charges, wherein coal is gasified in a fluidized bed while reducing the charge and sponge iron is melted and drawn off. To make things worse, such meltdown gasification reactions, generally, are optimizied with a view to the pig iron output desired such that no environmentally safe slag will be formed.
In earlier proposals made by the Applicant, methods of the initially defined kind for recovering pig iron and environmentally safe slags have already been described, in which a molten slag bath and a molten iron bath are used at a volume ratio of between 0.5:1 and 1.5:1 and carbon carriers are introduced into the bath while top-blowing hot air.
In addition to the processing of steel slags such as, for instance, electric furnace slags, the known methods, above all, aimed at processing converter slags and also oxygen was blown into the bath, if only facultatively. Compared to electric furnace slags, converter slags and, in particular, LD slags are characterized by a substantially higher basicity CaO/SiO2 of usually more than 3, whereas electric furnace slags have a slightly lower basicity. Considering the various options discussed in earlier proposals, the realization of the method in a converter was the primary objective, thus using a more or less complex blowing technology. Yet, in principle, that earlier method was, of course, suitable also for use in furnaces or ladles.
The present invention aims to improve the known methods for. use primarily with electric furnace slags and, at the same time, to get by with less apparative expenditure than according to the earlier proposals. To solve this object, the method of the initially defined kind essentially resides in that a mixed slag having a basicity CaO/SiO2 of between 1.2 and 2.5 is adjusted from the starting slags and iron carriers and the mixed slag is transferred into a hearth-type furnace or a ladle, that hot air is top-blown for afterburning at a post-combustion degree PC=(CO2+H2O)/CO+CO2+H2+H2O) of between 0.70 and 0.85 the CO formed from the carbon in the bath, and that the hot offgases from the furnace or the ladle are drawn off tangentially to the axis of the mouth of the hot air feeding lance. The use of furnaces or ladles enables the application of substantially more cost-effective devices in which also the refractory lining element of costs is substantially reduced. Taking into account that, in the instant case, a higher wear of refractory materials may be put up with without excessive investment costs, a substantially higher basicity of up to 2.5 is, thus, regarded as admissible in the method according to the invention such that, in particular, electric furnace slags may be employed directly without a lime batch or a remarkable lowering of the basicity. At the same time, it is, however, also feasible in the context of an electric steel making plant to ensure the perfect control of the waste streams occurring in electric steel making plants. There, besides electric furnace slag also ladle slags, dusts, millscale and furnace break-off material occur as waste substances, which may be disposed of or utilized immediately by the method according to the invention, whereby a favorably priced molten pig iron substantially free of trace elements will be obtained at the same time. In the method according to the invention by increasing post-combustion to values of between 0.7 and 0.85 and, in particular, values of around 0.8, the control of the method according to the invention has been substantially facilitated. It is merely required to care for the adequate supply of carbon into the bath and ensure the appropriate blending of the slag with the bath and subsequently afterburn the CO dissolved in the slag by the aid of hot air impinging at a high speed. Blowing oxygen or hot air into or through the bath may be obviated completely such that the apparative expenditure can be considerably reduced. Carbon basically may be fed via simple lances or via bottom tuyeres, which does not imply a substantial increase in the costs for the ladle or the furnace. Yet, also carbon may be blown into the bath through submerged lances. In a particularly advantageous manner, carbon also may be top-charged directly onto the melt bath in a coarse grained form, using a bottom-stirred ladle. What is, however, also essential to the efficiency of post-combustion, and hence the high heat transfer and the obviation of additional heating of the furnace or the ladle by the aid of external heating means, is the evacuation of the hot furnace offgases proposed by the invention. According to the invention, this evacuation is to be effected tangentially to the axis of the mouth of the hot air feeding lance. Such a gas flow control renders feasible to not only safely obtain the desired post-combustion degree, but also ensure the desired heat transfer such that a number of waste materials from electric steel making plants as initially described can be disposed of or utilized in a cost-effective manner.
Advantageously, the method according to the invention is carried out in a manner that the offgases drawn off are supplemented with additives selected from the group consisting of fine ores, dusts, millscale and optionally additional fuel in a further, external post-combustion course and the heated solids are blown onto the slag together with the hot air. In said post-combustion course, also waste streams from an electric steel making plant may obviously be melted at least partially, wherein, due to the adjustment of a post-combustion degree of between 0.7 and 0.85 within the furnace or ladle, sufficient chemical energy will still be contained in the offgases in order to ensure such an effective afterburning outside the furnace or ladle. After this, or alternatively, it may, however, also be proceeded in that the combustion offgases of the post-combustion course are conducted via a hot cyclone for separating the possibly liquefied solids and a heat exchanger for heating the hot air, thus attaining a particularly high energy yield.
The partially cold waste streams advantageously can be melted prior to charging. The additives are preferably melted in a melting cyclone and admixed in the liquid form to the basic slags such as, e.g., steel slags or electric furnace slags. The mixed slag is transferred into the furnace or the ladle and the hot O2-containing propellants of the melting cyclone are top-blown onto the slags as hot air.
An effective limitation of the post-combustion occurring within the furnace or the ladle and a suitable convection with a high heat transfer occurring in the interior of the gas space of the furnace or the ladle may be enhanced even further. A pressure reduced relative to the gas pressure prevailing within the gas space of the furnace or the ladle is maintained in the duct for the hot offgases from the furnace or the ladle.
Generally, the process control according to the invention primarily affords advantages in connection with the operation of an electric steel making plant, wherein concentrated zinc or zinc-oxide-containing products formed from occurring dusts may be further upgraded at place or directly delivered to metallurgical processors. The strongly basic metallurgical residual substances as well as break-off materials such as, e.g., furnace break-off material, may be converted into a high-quality clinker substitute and high-grade pig iron by the addition of favorably priced acidic fine ores. Advantageously, also blast furnace slags may be used for lowering the basicity of electric furnace or converter slags.